Azimuth measurement technology in downhole is mostly categorized into measurements with magnetometers and measurements with gyroscopes. The former uses triad magnetometers to measure earth magnet field. However, the magnetometers can be used only in the place without any magnetic influence by magnetic materials like a casing. The latter uses gyroscopes to measure the earth rate vector direction for azimuth determination. The gyroscope measurements are hardly influenced by magnetic field. Therefore, the gyroscope can be used near or in such a casing of magnetic materials.
To determine azimuth using the earth rate in downhole, three earth rate components are usually measured with three orthogonal axis gyroscopes. The three components can determine the earth rate vector in the sensor coordinates system in any attitude angle. If the two sensor axes lie on the horizontal plane, a ratio of the two components can determine north and thence azimuth. North is a projection direction of the earth rate vector onto the horizontal plane. If a single-axis gyroscope rotates its input axis on the horizontal plane about the vertical axis, it can measure two orthogonal components. In this case, it is not necessary even to calibrate the scale factor if only the direction of the projection vector is measured.
It is advantageous to use relatively low grade gyroscopes like MEMS (Micro-Electro Mechanical Systems) gyroscopes because of its low cost, small size and high environmental reliability. Today the performance of MEMS gyroscopes is approaching that of conventional high grade gyroscopes such as optical fiber gyroscopes. However, the MEMS gyroscopes show still too low bias stability to measure the earth rate in a strap-down configuration. To cancel out the bias error, the input axis direction needs to be flipped. If a flipping mechanism is also capable of pointing the input axis to various angular orientations, a single-axis gyroscope can determine two orthogonal earth rate components as described in U.S. Pat. No. 7,712,223 issued on May 11, 2010 and pending U.S. patent application Ser. No. 12/233,592 filed on Sep. 19, 2008. The U.S. Pat. No. 7,712,223 and U.S. patent application of No. 12/240,943 are incorporated herein by reference in its entirety.
The method using the flipping mechanism, however, requires a plane including two orthogonal axes to be kept horizontal. If the plane tilts, the relationship between the two measured earth rate components and north becomes nontrivial. Therefore, three orthogonal components of the earth rate vector must be measured to enable the measurement system to tilt at any direction and any angles. Such system needs more complex mechanism to flip the gyroscope for canceling the bias and/or to point the input axis to various angular orientations for measuring all the three earth rate components. The mechanism makes the sensor package larger and less reliable in spite of small size and high reliability of the MEMS gyroscope. The mechanism may also cause uncertain misalignment errors due to its mechanical tolerances. If only a single axis gyroscope suffices to determine azimuth, these disadvantages could be reduced and new applications would be expected. Therefore, it is beneficial if the restriction that the measurement plane must be kept horizontal is eliminated.